1. Field of the Invention
The present invention relates golf balls providing low spin rates and high launch angles to achieve increased ball flight distances.
2. Description of the Background Art
For golf balls, spin rate is an important factor having a significant influence on their flight performance and controllability. If a golf ball has a high spin rate, backspin allows the ball on the green to stop rapidly, and the ball can also be side spin and thus controlled so that its flight trajectory can draw or fade. As they are superior in controllability, golf balls of high spin rates are preferred by professional golfers and golfers with small handicaps.
A golf ball of a high spin rate, however, is not suitable for beginners or golfers with large handicaps, who cannot control the ball""s spin rate skillfully. When such a golfer hits a golf ball with a golf club the golfer unintentionally imparts sidespin to the ball. The ball is thus sliced or hooked and fails to fly in a direction as intended and also provides a reduced ball flight distance. As such, golfers with large handicaps prefer golf balls of low spin rates as such balls less slice and hook and thus provide large flight distances.
From the above view point, U.S. Pat. No. 5,368,304 discloses a technique related to a golf ball of a low spin rate. This technique provides a golf ball including a core having a Reihle compression of at least 0.076 and a cover having a Shore D hardness of at least 65. More specifically, the core can be relatively soft while the cover can be relatively hard to provide the golf ball with a low spin rate.
Conventional techniques employed to adjust spin rates are determined by the profile in rigidity of the entire golf ball, the magnitude in rigidity of the ball""s outermost layer, the thickness of the outermost layer, the profile in specific gravity of the entire ball, and many other factors combined organically. As such, a spin rate insufficiently reduces, or even if it does sufficiently, another factor having a significant influence on flight distance, or a launch angle, lowers and as a result flight performance insufficiently improves.
The present invention contemplates a golf ball further lower in spin rate than conventional golf balls of low spin rates and higher in launch angle to alleviate slice and hook and also further increase ball flight distances to be suitable for golfers having large handicaps. The present invention is based on a finding that relatively optimizing a dynamic rigidity provided in a direction of impact deform in a longitudinal direction and that provided in a direction in which the ball has torsion, can provide the ball with a low spin rate and a high launch angle.
The present invention is a golf ball providing a ratio of a primary natural frequency provided in a direction in which the ball deforms (in a longitudinal direction) (fn) and a primary natural frequency provided in a vibration mode in a direction of torsion (fc), i.e., a ratio (fc/fn) in a range:
2.22xe2x89xa6fc/fnxe2x89xa62.45. 
Herein, value (fc/fn) is preferably 2.26 to 2.42, more preferably 2.28 to 2.35.
To adjust value (fc/fn) to fall within the above range, the golf ball can include a solid core and a cover, the solid core being formed of a plurality of layers and having a center smaller in complex modulus than the core""s outermost layer.
In particular the solid core can effectively be formed of a plurality of layers having a complex modulus smallest at the center layer, larger at the outer layer(s) and largest at the outermost layer to achieve value (fc/fn) in the above range. Suitably the cover is greater in complex modulus than the core adjacent thereto.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.